MRI is primarily used to non-invasively render anatomical details for improved diagnosis of many pathologies and diseases (Sitharaman, B. & Wilson, L. J. Gadofullerenes and Gadonanotubes: A new paradigm for high-performance magnetic resonance imaging contrast agent probes Journal of Biomedical Nanotechnology 3, 342-352 (2007); Pan, D. et al. Revisiting an old friend: manganese-based MRI contrast agents. WIREs Nanomedicine and Nanobiotechnology 3, 162-173 (2010)). The development of MRI has led concurrently to increased use of chemical contrast-enhancement products called contrast agents (CAs) which improve detection of pathologic lesions by increasing sensitivity and diagnostic confidence.
The two main types are T1 and T2 MRI CAs, and affect (decrease) the longitudinal T1 and transverse T2 relaxation times of water protons, respectively. The quantitative measure of their effectiveness to accelerate the relaxation process of the water protons is known as relaxivity; the change in relaxation rate (inverse of relaxation time) per unit concentration of the MRI CA. The widely-used clinical T1 MRI CAs are mainly synthesized as metal-ion chelate complexes, where the metal ion is the lanthanoid element gadolinium (Gd3+), or the inner-transitional element manganese (Mn2+). A large body of experimental and theoretical research done in the last three decades now offers good understanding of the relaxation mechanism, and underlying structural, chemical and molecular dynamic properties that influence the relaxivity of these paramagnetic-ion chelate complexes (Aime et al., 1998, Chemical Society Reviews 27: 19-29; Caravan et al., 1999, Chem Rev 99: 2293-2352; and Lauffer, 1987, Chem Rev 87: 901-927). Theory suggests that the relaxivity of these MRI contrast agents is sub-optimal, and predicts the possibility of developing new contrast agents up to at least fifty to hundred times greater relaxivity (Merbach et al., 2001, The Chemistry of Contrast Agents in Medical Magnetic Resonance Imaging: John Wiley & Sons. 471; and Datta et al., 2009, Accounts Chem Res 42: 938-947).
Most clinical MRI CAs are paramagnetic T1-weighted CAs, which enhance MR signals to produce bright positive contrast such as gadolinium-(Gd3+) ion-based T1 CAs. The recent discovery of nephrogenic systemic fibrosis (NSF) in some patients with severe renal disease or following liver transplant has generated concern leading to Food and Drug Administration (FDA) restrictions on clinical use of Gd3+-ion based ECF MRI CA (Girdhar, G. & Bluestein, D. Biological Effects of Dynamic Shear Stress in Cardiovascular Pathologies and Devices. Expert Rev. Medical Devices 5, 167-181 (2008)).
Recently, the element manganese has received attention as a possible alternative to gadolinium. Manganese was reported early on as an example of paramagnetic contrast material for MRI. Unlike the lanthanides, it is a natural cellular constituent resembling Ca2+ and often functions as a regulatory cofactor for enzymes and receptors. Normal daily dietary requirement for manganese is 3-8 μmol while normal serum levels are 0.001 μmol/l. Manganese toxicity has only been reported following long-term exposure or at high concentrations resulting in neurological symptoms (Pan, D. et al. Revisiting an old friend: manganese-based MRI contrast agents. WIREs Nanomedicine and Nanobiotechnology 3, 162-173 (2010)).
Over the past 10 years, carbon nanostructures such as gadofullerenes (represented as Gd@C60 Gd@C80 and Gd@C82) and gadonanotubes (represented as Gd @US-tubes, where US-tubes=ultra-short SWNTs) that encapsulate Gd3+ metal ion have been proposed as T1 CAs for MRI (Sitharaman, B. & Wilson, L. J. Gadofullerenes and Gadonanotubes: A new paradigm for high-performance magnetic resonance imaging contrast agent probes Journal of Biomedical Nanotechnology 3, 342-352 (2007)). The synthesis strategies in the development of these complexes have focused on covalently or non-covalently functionalizing multiple Gd3+-chelate complexes onto the external carbon sheet of carbon nanostructures such as carbon nanotubes and nanodiamonds (Richard et al., 2008, Nano Letters 8: 232-236; and Manus et al., 2009, Nano Letters 10: 484-489), or encapsulation of Gd3+-ions within the carbon sheet of carbon nanostructures such as fullerene (a.k.a. gadofullerenes) (Toth et al., 2005, J Am Chem Soc 127: 799-805; Kato et al., 2003, J Am Chem Soc 125: 4391-4397; and Fatouros et al., 2006, Radiology 240: 756-764), and single-walled carbon nanotubes (a.k.a. gadonanotubes) (Sitharaman et al., 2005, Chem Commun: 3915-3917; and Ananta et al., 2010, Nature nanotechnology 5: 815-821). These Gd3+-ion carbon nanostructures show between two-fold to two-order increase in relaxivity (depending on the magnetic field) compared to Gd3+-chelate complexes with the gadonanotubes showing the highest relaxivities at low to high (0.01-3 T) magnetic fields. However, the potential and efficacy of Mn2+-ion carbon nanostructure complexes as MRI CAs still has not been investigated.
The variable-magnetic field (0.01-3 T) relaxivity or nuclear magnetic resonance dispersion (NMRD) profiles of the gadonanotubes are characteristically different than those obtained for any other MRI CA and their relaxation mechanisms are not well understood. A major reason for this lack of understanding is that unlike Gd3+ ion chelates, which can be prepared at a very high level of purity and unambiguously characterized, the carbon nanostructure-Gd3+ ion systems are rather complex mainly due to their particulate nature, and intricate relationships linking their chemical, geometric, and magnetic characteristics to their properties as MRI contrast agents. Nevertheless, geometric confinement of the Gd3+ ion within nanoporous structures maybe one reason (Ananta et al., 2010, Nature nanotechnology 5: 815-821; and Bresinska I, 1994, J Phys Chem 98: 12989-12994). While confinement of the Gd3+ ions into nanoporous structures of silicon (Ananta et al., 2010, Nature nanotechnology 5: 815-821) or zeolites (Bresinska I, 1994, J Phys Chem 98: 12989-12994) increases the relaxivity by two or four times compared to Gd3+ chelate compounds, only when the Gd3+ ion are confined within single-walled carbon nanotubes (Sitharaman et al., 2005, Chem Commun: 3915-3917; and Ananta et al., 2010, Nature nanotechnology 5: 815-821) has there been an order of magnitude or more increase in relaxivity (irrespective of the magnetic field strength) with NMRD profiles significantly different that those reported for other Gd3+ ion-based complexes. Additionally, to date, there have been no studies performed to systematically investigate whether the high increase in relaxivity and unconventional NMRD profiles are unique to paramagnetic ions confined in single-walled carbon nanotubes, which are seamless cylinders formed from a graphene sheet, or in general observed for paramagnetic ions confined in other graphene or graphitic structures.
Graphene, a two-dimensional (2-D) nanostructure of carbon, has attracted a great deal of attention showing potential for various material and biomedical science applications (Novoselov, K. S. et al. Electric field effect in atomically thin carbon films. Science 306, 666 (2004)). Theoretical studies predict a variety of magnetic phenomena in graphene (Makarova, 2004, Semiconductors 38: 615-638), and to date, few of these effects have been explored experimentally (Wang, et al., 2008, Nano Letters 9: 220-224). Recently, simple potassium permanganate (KMnO4)-based oxidative chemical procedures have been used in the large scale production of graphite oxide, graphene nanoplatelets, and graphene nanoribbons using starting materials such as graphite and MWCNTs (Stankovich, et al., 2007, Carbon 45: 1558-1565; and Kosynkin, et al., 2009, Nature 458: 872-876). In this work, experimental studies were performed to characterize the physico-chemical properties of graphite oxide, graphene nanoplatelets, and graphene nanoribbons synthesized using modified protocols of these techniques. We demonstrate that trace amounts of Mn2+ ions get confined (intercalated) within the graphene sheets during the synthesis process, and that this confinement in general substantially increases the relaxivity (up to 2 order) compared to paramagnetic chelate compounds, and these materials show diverse structural, chemical and magnetic properties with NMRD profiles different than those of the paramagnetic chelates.
Recent reports have shown that affordable large scale production of graphene nanoplatelets (GNPs) and graphene nanoribbons (GNRs) is possible by using chemical techniques (Stankovich, S. et al. Stable aqueous dispersions of graphitic nanoplatelets via the reduction of exfoliated graphite oxide in the presence of poly (sodium 4-styrenesulfonate). Journal of Materials Chemistry 16, 155-158 (2006); Stankovich, S. et al. Synthesis of graphene-based nanosheets via chemical reduction of exfoliated graphite oxide. Carbon 45, 1558-1565 (2007); Stankovich, S., Piner, R., Nguyen, S. & Ruoff, R. Synthesis and exfoliation of isocyanate-treated graphene oxide nanoplatelets. Carbon 44, 3342-3347 (2006); Li, D., Müller, M., Gilje, S., Kaner, R. & Wallace, G. Processable aqueous dispersions of graphene nanosheets. Nature nanotechnology 3, 101-105 (2008); Kosynkin, D. et al. Longitudinal unzipping of carbon nanotubes to form graphene nanoribbons. Nature 458, 872-876 (2009); Higginbotham, A., Kosynkin, D., Sinitskii, A., Sun, Z. & Tour, J. Lower-Defect Graphene Oxide Nanoribbons from Multiwalled Carbon Nanotubes. ACS nano 4, 2059-2069 (2010); Geng, Y., Wang, S. & Kim, J. Preparation of graphite nanoplatelets and graphene sheets. Journal of colloid and interface science 336, 592-598 (2009)).